Method for compensating for bottom warpage of a BGA integrated circuit

ABSTRACT

An integrated circuit includes a plurality of solder balls arrayed on the bottom surface of a package of the integrated circuit. These solder balls provide for surface mounting of the integrated circuit to a circuit board by solder reflow. The array of solder balls can be planarized so that each of the plural solder balls participate in defining a truly planar solder ball contact array for the integrated circuit package. Methods of manufacturing the integrated circuit with a package having planarized solder balls in an array dependent from a bottom surface thereof are set forth. The truly planarized solder ball contact array of the integrated circuit package affords nearly absolute reliability in forming of surface-mount electrical connections between the integrated circuit package and the circuit board on which the package is to mount. Additionally, the planarized solder ball contacts locally compensate individually for warpage of the integrated circuit package by variation in the individual dimensions of dependency of each solder ball below the bottom surface of the package.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 08/506,382, filed Jul. 24,1995, now U.S. Pat. No. 5,745,986 which is a divisional of Ser. No.08/192,081, filed Feb. 4, 1994, which issued as U.S. Pat. No. 5,435,482on Jul. 25, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of integrated circuits. Moreparticularly, the present invention is in the field of integratedcircuits of surface-mount technology type, having a package which housesthe semiconductor chip and which provided electrical interface from thischip to plural electrical contacts disposed in an array on the bottom ofthe package. The chip is mounted to a circuit board, for example, bysurface mounting its package in electrical connection with a congruentarray of electrical contacts on the circuit board. Generally, aplurality of solder balls at the congruent contacts along with a solderreflow operation is used to both electrically connect the contacts ofthe integrated circuit package with the congruent contacts of thecircuit board, and to physically mount the integrated circuit package onthe circuit board.

2. Related Technology

Conventional surface-mount type of integrated circuits are known inwhich a ceramic package houses the integrated circuit, and which have aplurality of electrical contacts on the lower surface of the package.These electrical contacts are connected to congruent contacts of acircuit board by use of a corresponding plurality of solder ballsdepending from the contacts of the package. These solder balls aregenerally all of the same size. For example, the solder balls may beabout 0.030 inches in diameter as set on the package bottom surface. Thecircuit board to which the package is to be mounted generally has acongruent array of electrical contacts at which a correspondingplurality of pads of solder paste (solder particles in paste flux) areapplied. When the integrated circuit package is placed on the circuitboard, the solder balls individually contact the pads of solder paste.Next, the circuit board and integrated circuit are placed in a solderreflow furnace, at a temperature and for a time sufficient to reflow thesolder balls and to coalesce the solder particles of the solder pasteinto a unitary solder connection with the solder balls of the integratedcircuit package.

Ceramic packages of the type described above generally are very stableand predictable dimensional. That is, the typical ceramic package mayhave an non-planarity of its bottom surface of no more than 0.001 to0.002 inches across a typical package dimension of 1.125 to 1.5 inchessquare. Consequently, when the solder balls are placed on the bottomsurface of such a package, they also define a mutual solder ball arrayplane which is non-planar by about the same amount as the package. Thatis, the solder balls themselves are generally made by a process similarto the making of buckshot, for example, which produces solder balls witha diameter dimension variation of no more than about +/-0.002 inches, orby a screen printing process using solder paste which is then reflowedto produce solder balls with a slightly higher degree of dimensionalvariability. With all of the conventional methods of making and placingthe solder balls on a package bottom surface, the combination of packageand solder ball dimensional variation creates an non-planarity of about0.004 to 0.005 inches at most. The conventional furnace solder reflowmounting and electrical connection method described above will achievereliable mounting and electrical connections with this degree ofdimensional variation. Such reliability in the mounting and electricalconnection of the integrated circuit packages to their mounting boardsis important because the solder joints between the contacts of thepackage and those of the circuit board are highly difficult to visuallyinspect non-destructively once the package is in place on the circuitboard. Statistical methods of quality control along with destructivetesting methods must be relied upon to provide confidence in the goodphysical securing of any particular package to its circuit board alongwith reliable electrical connection of each of the package contacts toits corresponding contact of the circuit board.

Conventional surface-mount type of integrated circuit packages are alsoknown which are fabricated of plastic rather than ceramic material.These plastic packages for integrated circuits are advantageously lessexpensive to manufacture, and have other advantages over the ceramicpackages described above. However, the plastic integrated circuitpackages also have serious disadvantage because the plastic material ofthese packages is not as dimensional stable as the more conventionalceramic material of the packages described above. That is, during themanufacture of these plastic packages, which is generally accomplishedwith an injection molding or plug molding type of manufacturing, thebase portion of the package may warp out of planarity. That is, the baseportion of the package which carries the depending solder balls may notbe flat. If this base portion of the integrated circuit package is bowedor wavy, for example, then the solder balls depending from this surfacewill define cooperatively a solder ball contact array plane which isalso bowed or wavy.

By careful consideration of the events taking place during the solderreflow operation to mount and electrically connect such a package to acircuit board, an appreciation of the problem outlined above may begained. When an integrated circuit package with its depending array ofsolder balls is placed on a circuit board, which has been prepared withpads of solder paste (solder particles in flux) on the contacts of thecircuit board, of the package and circuit board are sufficiently planar,than each of the solder balls will contact its corresponding pad ofsolder paste. Subsequently, when the circuit board and circuit packageare heated in the solder reflow furnace, the reflowed solder balls andcoalescing solder paste will join by mutual surface tension.

However, if the integrated circuit package is sufficiently bowed or wavythat some of the solder balls do not contact their corresponding solderpaste contact pads, then mutual surface tension is not developed atthese locations. When the solder paste and solder balls which are notcontacting one another are reflowed, the surface tension of each tendsto make them flatten but along their respective surfaces, and actuallydraw away from one another. Consequently, an electrical connection isgenerally not effected at the solder balls which do not contact theircorresponding pads of solder paste. Also, the solder balls which do notelectrically contact their corresponding circuit board contact do notparticipate in the physical securing of the package to the circuitboard. Moreover, the physical mounting of the package to the circuitboard is also compromised when solder balls do not join with theircorresponding solder paste pads.

A contemporary plastic integrated circuit package may have a bottomsurface non-planarity of about 0.004 inches. When solder balls are addedon this bottom surface, and contribute their own dimensional variabilityof 0.002 inches, or slightly more, then the total non-planarity to beexpected at the solder ball array plane is at least 0.006 inches. Thisnon-planarity is at the limit of what can be tolerated, and in factstatistically decreased the reliability of the electrical connectionsbetween such a package and a circuit board upon which it is to bemounted. In fact, a joint industry council, JEDEC, has proposed anindustry standard for the planarity of the solder ball contact array atthe bottom surface of plastic integrated circuit packages to addressthis problem. This standard calls for a non-planarity of no more than0.006 inches.

SUMMARY OF THE INVENTION

In view of the deficiencies of current surface-mount technologyintegrated circuits, the present invention has as a primary object toprovide such an integrated circuit housed in a package which has anarray of dependent solder ball contacts in an array on the bottomsurface of the package, and which solder balls cooperatively define asolder ball contact array plane of substantially perfect planarity.

More particularly, the present invention has as an object to providesuch an integrated circuit with a plastic package which may be warped,for example, and not provide a perfectly planar surface on its bottomside, and yet which carries an array of solder ball contact members onits bottom side which cooperatively define a substantially planer solderball contact array.

An additional object of the present invention is to provide a method ofmaking such an integrated circuit.

Accordingly, the present invention provides an integrated circuitincluding a package housing an integrated circuit chip and providing forenvironmental protection of the circuit chip as well as electricalinterface of the circuit chip with external electrical circuitry andphysical mounting of the package to a circuit board, for example, andwhich package carries a dependent array of solder ball electricalcontact members cooperatively defining a solder ball contact array planeof substantial planarity.

Still further, the present invention provides a method of making anintegrated circuit including the steps of providing a plurality ofsolder ball contact members dependent from a bottom surface of a packageof the integrated circuit to cooperatively define a solder ball contactarray plane, and engaging at least some individual ones of said solderballs in said plurality of solder ball contacts with a planar member tomake said solder ball contact array plane substantially planar.

Additional objects and advantages of the present invention will beapparent from a reading of the following detailed description of asingle exemplary embodiment of the present invention; and of a method ofmaking the integrated circuit including two alternative ways ofplanarizing the plurality of solder ball contacts cooperatively formingthe solder ball contact array plane at the bottom surface of theintegrated circuit, all taken in conjunction with the appended drawingFigures in which like reference numerals designate the same feature, orfeatures which are analogous in structure or function.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a plastic package surfacemount integrated circuit assembly with an integrated circuit chip andelectrical interconnection (TAB) device;

FIG. 2 is a view similar to FIG. 1, but showing the bottom surface ofthe integrated circuit package;

FIG. 3 is an enlarged fragmentary cross sectional elevation view of aportion of the integrated circuit seen in the preceding drawing figures,and illustrating a result of the manufacturing step depicted in FIG. 4;

FIG. 4 provides an enlarged elevation view of the integrated circuitpackage as it would appear at a particular stage of manufacture, andschematically depicts a manufacturing tool which is used at this stageof the manufacturing process; and

FIG. 5 provides a schematic cross sectional elevation view of anintegrated circuit according to the present invention preparatory tomounting and electrical connection of the integrated circuit to acircuit board.

DESCRIPTION OF THE PREFERRED EXEMPLARY

Embodiment of the Invention

Viewing FIGS. 1 and 2 in conjunction, a surface-mount technologyplastic-package ball-grid array integrated circuit assembly 10 isdepicted in exploded view. The integrated circuit assembly 10 includes atwo-part package, generally indicated with the numeral 12. This package12 includes a base part member 14, which is a generally planar body ofplastic material which is square in plan-form, and which defines arecess 16 into which an integrated circuit chip 18 is receivable.Surrounding the recess 16, the base member 14 carries a plurality ofelectrical contacts 20. These contacts 20 each individually connectelectrically to corresponding ones of plural electrical contacts 22disposed on the bottom surface 24 of the base member 14, also as is seenin FIG. 2. The electrical contacts 22 may be essentially flush with thebottom surface 24 of the package 12, or may protrude or be recessedslightly.

However, carried on the bottom surface 24 of the base member 14 is afilm or coating of solder resist material 26. This solder resistmaterial defines plural openings 28 corresponding to the plurality ofcontacts 22. In other words, each of the plural contacts 22 is disposedin a corresponding one of the plural openings 28, and is surrounded bythe coating or film 26 of solder resist material to be separated fromadjacent contacts by an interposed portion of the solder resist film orcoating.

As will be seen, the package 12 receives the chip 18 into the recess 16,there to be secured by an adhesive material (not seen in the drawingFigures). Viewing FIG. 1, it is seen that the chip 18 includes pluralperipherally-disposed fine-dimension electrical contacts, generallyreferenced with the numeral 30. Each of the individual contacts of theplurality of contacts 30 finds correspondence in an individual contactof the plurality 20. In order to effect electrical connection betweenthe contacts 22 and 30, the integrated circuit assembly 10 includes athin rectangular TAB (tape automated bonding) member 32. This TAB member32 includes a thin film 34 which carries an array of electrical contacts36 surrounding a central opening 38. This central opening 38 correspondsto the central active circuit area 40 of the integrated circuit chip 18.The arrangement of the contacts 36 matches that of the array of contacts30 on the chip 18.

Similarly, the film 34 carries a peripheral array of contacts 42matching the array of contacts 20 on the base member 14. The contacts 36and 42 are individually connected correspondingly with one another by aplurality of conductive traces 44 extending therebetween across the film34. When the film 34 is placed congruently over the base 14 andintegrated circuit chip 18 in recess 16, the contacts 36 connect withcontacts 30, and contacts 42 connect with contacts 20. The film 34 isself-adhesive or is secured in place with a separate adhesive. Theelectrical connection of the contacts 36 and 42 of TAB 38 with thecontacts of the chip and base member may be accomplished with simplesurface-contact electrical conduction between these contacts, or mayalso be effected with a solder reflow step, for example. Alternatively,an ultrasonic pressure-bond process may be used to effect secureelectrical connection between the TAB 32 and each of the circuit chip 18and base part member 14. The use of TAB interconnection or electricalinterfacing technology is well-known in the semiconductor integratedcircuit packaging art.

Still viewing FIG. 1, it is seen that the package 12 includes a coverportion 46 which sealingly cooperates with the base part member 14 toprovide a sealed and protected environment for the integrated circuitchip 18. During manufacturing of the integrated circuit assembly 10, thepackage cover portion 46 is sealingly attached to the base part member14, for example, by use of a potting compound or epoxy. As a result, theintegrated circuit chip 18 is environmentally protected once themanufacturing operations are completed to install the chip in thepackage 12 and to close this package. Alternatively, the cover portion46 may be replaced with a simple layer of epoxy or other pottingcompound to provide environmental closure for the package 12, and forthe integrated circuit chip 18 therein.

In order to provide for electrical connection and simultaneous physicalmounting of the integrated circuit assembly 10 to a circuit board (seenin FIG. 5), the base part member 14 carries a plurality of solder ballelectrical contact members 48, each disposed upon and electricallyconnecting to a corresponding one of the plural electrical contacts 22.FIG. 2 depicts that these solder ball members 48 may be formed asseparate elements which are then placed into the openings 28 of solderresist film 26. The solder balls 48 may be secured by a solder reflowoperation, for example, in which the material of the solder ball itselfis melted to secure the ball to the contact 22. Alternatively, a layerof lower melting temperature solder may be provided in the openings onthe contacts 22, and may be melted to secure the solder balls 48 withoutmelting of these solder balls themselves. Still alternatively, solderpaste (solder particles in a paste flux) may be screen printed onto thecontacts 22 within the openings 28. This solder paste is then coalescedinto a solder ball by furnace heating. Surface tension causes the soldermaterial to stand up above the surrounding surface of the material 26somewhat like a ball. Regardless of the method used to provide thedepending protruding solder electrical contact members 48, these contactmembers 48 are herein referred to as solder "balls".

As will be further seen viewing FIG. 3, the electrical contacts 22 aredefined by the end surface portions 50 of plural correspondingelectrically conductive vias 52 extending through the material of thebase part member 14 between the contact features 20 and the bottomsurface 24 of this package base part member 14. The solder balls 48secure to the contacts 22 (end surface portions 50) of vias 52 withinthe openings 28 of solder resist film 26. FIG. 3 further shows that thesolder balls 48 individually may not each depend an equal distance (inthe negative X direction) below the surface 24. That is, the two solderballs 48' in FIG. 3 may depend further below the surface 24 than doesthe other solder ball 48".

Additionally, when the entire plurality of solder balls 48 on the bottomsurface 24 of the package 12 is considered, there may be a considerablenon-planarity to a virtual surface or solder ball contact array plane 54cooperatively defined by the solder balls 48. In part, thisnon-planarity of the contact array plane 54 is due to the non-uniformdistance of dependance of the individual solder balls 48, and is alsodue to the possible warpage or waviness of the bottom surface 24 of thepackage 12 itself. The scale of FIG. 3 is such that this package warpageis not visible, but it may be easily envisioned by viewing the structureof FIGS. 1 and 2 and imagining the spherical, cylindrical, conical, orwavy, for example, distortions of this package structure at the surface24 which could occur due to warpage and distortion of the plasticmaterial of the package 12. Also, some non-planarity at the solder balls48 may result from slight protrusions or recessing of the contactsurface (end surface 50) of the vias 52 relative to the surface 24. Thisoverall non-planarity of the contact array plane 54 is shown in FIG. 3by the cranking of this plane or virtual surface over the center contact48'.

In order to planarize the contact array plane 54, and provide anon-uniform dimension of dependence of the individual solder ballcontacts 48 so as to locally compensate for any warpage of the package12 at its bottom surface 24, the integrated circuit assembly 10 istouched to a planarizing tool 56 shown in FIG. 4. The planarizing tool56 may take the form of a strong platen block which is supported so asto sustain forces applied thereto, as will be explained. This platenblock 56 has a planar upper surface 60 which is strong and stable so asto provide a reference plane to which the plural solder balls of thesolder ball contact array on the integrated circuit 10 will be matched.

A vacuum chuck pressing tool 62 is used to hold the integrated circuitassembly 10, and to forcefully engage this circuit assembly 10 with thesurface 60 of platen 56. The vacuum chuck tool 62 includes a chuckmember 64 with a foot portion 66 configured at a peripheral edge surface68 to engage and support the integrated circuit assembly 10. A cushion70 of yieldable material is interposed between the peripheral edgesurface 68 and the integrated circuit assembly 10 to both form a vacuumseal and to yieldably support the integrated circuit package 12. A stemportion 72 extends upwardly from the foot portion 66 to a ball portion74. These stem and ball portions 72 and 74 cooperatively define athrough passage 76 communicating with a vacuum chamber 78 defined withinthe foot portion 66 cooperatively by the cushion member 70 andintegrated circuit assembly 10.

A ball socket portion 80 of the chuck and pressing tool 62 defines apart spherical recess 82 movably but sealingly receiving the ballportion 74. A pressing ram 84 carries the ball socket portion 80 and iseffective to move vertically and forcefully engage the integratedcircuit 10 with the surface 60, as is indicated by arrow 86. Ram 84 andsocket portion 80 cooperatively define a through passage 88 connectingwith passage 76 to communicate chamber 78 with a source of vacuum, asindicated by arrow 90.

When the ram 84 is lowered to forcefully engage the integrated circuit10 at its plurality of solder ball contacts 48 with the surface 60, thefoot portion 66 may pivot on ball portion 74 (as depicted by arrow 92)to ensure uniform distribution of force over the area of the array ofsolder ball contact 48. As FIG. 3 depicts, the solder balls 48 mayindividually be deformed so that they cooperatively define a solder ballcontact array plane 94 which is truly planar. In other words, solderball contacts like those indicate with 48' at the left side of FIG. 3will be distorted force the material indicated at 96 upwardly and todecrease their dimension of dependence from the surface 24. On the otherhand, those solder balls like ball 48" at the right side of FIG. 3,which are somewhat more plump than the solder balls 48', and which donot depend as far even though containing substantially the same amountof solder, are substantially not changed by the pressing process. Thatis, the amount of force applied is chosen to be just sufficient to"flatten" the highest solder balls and bring the contact plane 94 to thehighest level of the lowest of the solder balls 48. In this way, thesolder balls 48 are also selectively distorted as needed to compensatelocally for any warpage of the package 12. The net result is anintegrated circuit assembly with a package 12 having a solder ballcontact array of mutually coplanar solder balls. Because each individualsolder ball 48 of the array is mutually coplanar in substantial effectwith all of the other solder balls, all of the solder balls 48participate at their point or area of maximum dependency from thesurface 24 in defining the contact plane 94. That is, none of the solderballs 48 crosses the contact plane 94, and none of the solder balls isleft out of or does not participate in defining the contact plane 94because of not extending far enough from the surface 24, in substantialeffect.

FIG. 4 also shown that the platen member 56 may be heated. That is theplaten 56 may include a controlled heat source, for example, as isindicated by the schematic electrical resistance heating element 96.This heating element is connected by electrical leads 98 to a controlledvoltage V_(c) so that a precisely regulated temperature may be achievedat the surface 60 of the platen member 56. By selecting a temperaturefor platen 56 and a residence time during which the integrated circuitassembly 10 is in contact with the platen such that the solder balls 48are locally softened but not liquified throughout their volume, theplural solder balls of the contact array may be planarized much asdescribed above. The surface 60 of platen 56 must be such that thesolder of solder balls 48 does not wet this surface. Thus, a planarizedarray of solder balls is achieved much as is depicted in FIG. 3.Advantageously, in this way the amount of force necessary to be appliedto the package 12 of the integrated circuit 10 is greatly reduced. Infact, with the proper selection to temperature and residence time, theapplied force may be minimal.

Attention now to FIG. 5 will quickly show how a great advantage isrealized from the present invention. FIG. 5 shows an integrated circuitassembly 10 preparatory to its setting upon a circuit board 100 havingthereon a plurality of electrical contacts (indicated generally witharrow 102), and each having a pad of solder paste 104 disposed thereon.The package 12 of integrated circuit 10 is seen to be bowed. If thesolder balls 48 of this package were not planarized, only a few of theouter ones of these plural solder balls 48 would contact the pads 104 ofsolder paste and form mutual surface tension therewith. However, thecentral contacts where the electrical contacts would be absent would notbe visible at all because they are near the center of the contact array.

Fortunately, the integrated circuit 10 has had its solder ball contacts48 planarized according to the present invention. Consequently, thesolder balls 48 cooperatively define a contact plane 94 which is trulyplanar in substantial effect. The planarized solder ball contacts 48cooperatively define the contact plane 94, and also locally compensateby individual and various dimensions of dependency from the bottomsurface 24 of the package 12 for any warpage of this package. Individualvariations in the original dimensions of the solder balls 48 is alsocompensated. Thus, when the integrated circuit 10 is placed on the padsof solder paste 104, all of the solder ball contacts 48 engage withtheir corresponding one of the plural solder paste pads 104 and form amutual surface tension therewith. When the circuit board 100 andintegrated circuit 10 are placed in a solder reflow oven, the solderballs 48 and contacting solder paste pad 104 coalesce into a unitarysolder connection between individual ones of the circuit board contacts102 and corresponding ones of the contacts 22 of the integrated circuitpackage 12.

While the present invention has been depicted, described, and is definedby reference to particularly preferred embodiments of the invention,such reference does not imply a limitation on the invention, and no suchlimitation is to be inferred. The invention is capable of considerablemodification, alteration, and equivalents in form and function, as willoccur to those ordinarily skilled in the pertinent arts. The depictedand described preferred embodiments of the invention are exemplary only,and are not exhaustive of the scope of the invention. Consequently, theinvention is intended to be limited only by the spirit and scope of theappended claims, giving full cognizance to equivalents in all respects.

We claim:
 1. A method of compensating for base member bottom surfacewarpage of a plastic package ball-grid array integrated circuit,comprising:providing a reference plane surface for a plurality of solderball electrical contact members depending downwardly from the basemember bottom surface of the plastic package ball-grid array integratedcircuit; said plurality of solder ball electrical contact membersdefining a non-planar solder ball contact array plane; heating saidreference plane surface to a sufficient temperature to cause individualones of said plurality of solder ball electrical contact members tolocally soften but not to such a sufficient temperature to causeindividual ones said plurality of solder ball electrical contact membersto liquify throughout their volume when they engage said reference planesurface and are held in contact therewith for a residence time period;bringing said plurality of solder ball members into engagement with saidreference plane surface to apply a sufficient force to a highest solderball member in said plurality of solder ball members to cause it toflatten when held in contact with said reference plane surface for saidresidence time period; and said sufficient force being only sufficientto bring said reference plane surface to a highest level of a lowestsolder ball member in said plurality of solder ball members toselectively distort the solder ball members to compensate locally forany warpage in said plastic package ball-grid array integrated circuit.2. A method of compensating for base member warpage of a plasticball-grid array integrated circuit, comprising:providing a referenceplane surface for a plurality of electrical contacts on the integratedcircuit; bringing said plurality of contacts into engagement with saidreference plane surface to cause at least one of the contacts to beflattened; and applying a sufficient force to said at least one of thecontacts to flatten it so said reference plane surface is brought to thehighest level of the lowest contact in said plurality of contacts toselectively distort the contact to compensate for any warpage in theplastic ball-grid array integrated circuit.
 3. A method of compensatingaccording to claim 2 wherein said contacts are plastically deformablecontacts.
 4. A method of compensating according to claim 2 wherein saidcontacts are solder balls deformable by applied heat.
 5. A method ofcompensating according to claim 4 further comprising:heating saidreference plane surface to a sufficient temperature to cause individualones of said plurality of solder ball electrical contact members tolocally soften but not to such a sufficient temperature to causeindividual ones said plurality of solder ball electrical contact membersto liquify throughout their volume when they engage the heated referenceplane surface and are held in contact therewith for a residence timeperiod.
 6. A method of compensating according to claim 5 wherein saidstep of bringing includes:bringing said plurality of solder ball membersinto engagement with said reference plane surface to apply a sufficientforce to a highest solder ball member in said plurality of solder ballmembers to cause it to flatten when held in contact with said referenceplane surface for said residence time period.
 7. A method ofcompensating according to claim 2 further comprising:mounting theplastic ball-grid array integrated circuit in a chuck to align thebottom surface thereof with said reference plane surface; and movingsaid chuck toward said planar surface a sufficient distance to bring thecontacts into proximate engagement with said reference plane surface. 8.A method of compensating according to claim 7 wherein:said chuck is avacuum chuck member; and wherein said step of bringing includes:engagingsaid circuit with said vacuum chuck; and applying a vacuum to saidvacuum chuck to releasably adhere said circuit thereto.
 9. A method ofcompensating according to claim 8 wherein said step of engagingincludes:providing said vacuum chuck with a ball socket joint.